Coaxial horn antenna

ABSTRACT

A wide band, multi-mode antenna having a plurality of coaxial, independently fed, radiating horns. Each horn has multiple feeds which can be energized in various phase relationships to control polarization. The antenna can be used as a direct radiator, or to illuminate a reflector, has transmit or receive capabilities, and is adaptable to monopulse operation. The antenna is a compact rigid unit of very simple construction.

United States Patent Walters et al. Feb. 4, 1975 [5 COAXlAL HORN ANTENNA3,325,817 6/1967 Ajioka et al. 343/786 [75] Inventors: Glenn A. Walters,Poway; Edward $22 E. VOnKline, La Mesa; Duane Tubbs, San Diego, all ofCalif. Primary ExaminerEl1 Lieberman Asslgneei g lp? Corporatlon, SimDiego Attorney, Agent, or FirmBrown & Martin [22] F11ed: June 18, 1973[57] ABSTRACT [21] Appl. No.: 370,612

A wide band, multi-mode antenna having a plurallty of coaxial,independently fed, radiating horns. Each horn [52] US. Cl. 343/778,343/786 h m lti le f eds which can be energized in various Illt. Cl.phase relationships [0 control pola izatio The an- Fleld of Search 786,tenna can be used as a direct radiator, or to illuminate 343/354 areflector, has transmit or receive capabilities, and is adaptable tomonopulse operation. The antenna is a References Clted compact rigidunit of very simple construction. UNITED STATES PATENTS 3,086,203 4/1963Hutchison 343/786 1 9 Draw"; Fgures PATENTEUFEB SHEET 1 [1F 2 PAIENTEBFEBI' 4|975 VERTICAL POLARIZATION HORIZONTAL COMPONENTS CANCEL Fig. 6 a

MONOPULSE DIFFERENCE PATTERN POLARIZATION:

AZIMUTH PLANE HORIZONTAL ELEVATION PLANE VERTICAL Fig.6 0

SHEET 2 [IF 2 PHASE PHASE SHIFTER I SHIFTER HYBRID HYBRID so PHASE PHASESHIFTER SHIFTER I 58-54 in s e-54in HORIZONTAL POLARIZATION VERTICALCOMPONENTS CANCEL Fig.'6b

MONOPULSE DIFFERENCE PATTERN POLARIZATION:

AZIMUTH PLANE VERTICAL ELEVATION PLANE HORIZONTAL Fig. 6d

1 COAXIAL HORN ANTENNA BACKGROUND OF THE INVENTION Wide band antennasusually involve the use of angular or periodic elements, such as cones,spirals and the like. These have limited power capability and are notreadily adaptable to polarization changes. The antenna elements areoften mounted on some type of supporting structure, resulting in a bulkyunit.

SUMMARY OF THE INVENTION The antenna described herein is very compactfor the range of frequencies it is capable of handling, and is a ruggedself-supporting structure. Multiple tubular elements are assembledconcentrically to provide stepped coaxial horns covering severalfrequency ranges. Each horn has feeds positioned orthogonally relativeto the horn axis, with means for energizing the feeds in selected phaserelationships to obtain the required polarization characteristics. Thehorns are essentially waveguide elements of circular or other suitablecross section, and act in combination in their concentric arrangement toprovide coaxial waveguides with substantially equal electricaldimensions. The radiating patterns are thus nearly constant as afunction of frequency and the phase center is substantially constantover the entire band of coverage. All the radiating apertures are nearplanar, but retain good isolation between apertures.

The primary object of this invention, therefore, is to provide a new andimproved coaxial horn antenna.

Another object of this invention is to provide a new and improvedco-axial horn antenna having concentrically assembled tubular waveguideelements forming stepped coaxial radiating apertures.

Another object of this invention is to provide a new and improvedco-axial horn antenna in which each horn has feed elements which can beenergized in selected phase relationships.

A further object of this invention is to provide a new and improvedco-axial horn antenna having wide band, multi-mode capability.

Another object of this invention is to provide -a new and improvedco-axial horn antenna which is compact and of rigid structure, adaptableto a variety of installations.

Other objects and many advantages of this invention will become moreapparent upon a reading of the following detailed description and anexamination of the drawings, wherein like reference numerals designatelike parts throughout and in which:

FIG. I is a perspective view ofa typical configuration of the antenna.

FIG. 2 is a side elevation view, partially cut away.

FIG. 3 is a sectional view taken on line 33 of FIG.

FIG. 4 is a side elevation view of the antenna mounted on a reflector.

FIG. 5 is a diagram of a typical arrangement of feed connections to theantenna.

FIG. 6a 6d are diagrams of the polarization characteristics of theantenna with various phase relationships of the feeds DESCRIPTION OF THEPREFERRED EMBODIMENT The antenna as illustrated in FIGS. 1-3 is typical,and

comprises three cylindrical horns l2, l4 and 16 of cylindricalconfiguration, mounted together in coaxial alignment. The horns areprogressively sized to provide an inner radiating aperture 18, aconcentric intermediate aperture 20 and a concentric outer aperture 22at the front end of the antenna. Depending on the particular use of theantenna, any reasonable number of horns may be used and the crosssection of the tubular elements need not be circular.

The rear of the antenna is closed by an end plate 24 and the forward endis held in coaxial alignment by dielectric closure elements. Theseinclude a disc 26 inset in the end of inner horn 12, a ring 28 betweenhorn l2 and intermediate horn l4, and a ring 30 between horn l4 andouter horn 16. In addition to providing support and sealing the hornsfor protection, the dielectric elements provide electrical isolationbetween the radiating apertures. Beamwidth is controlled by staggeringthe forward ends of the horns, with the inner horn projecting furthest,the staggering also adding to the isolation. Beamwidth is reduced as thestagger is increased and can be set to suit specific characteristics.For some uses it may be desirable to enclose the front of the antenna bya dielectric radome or cover 32, indicated in broken line in FIG. 2.

In the structure illustrated, the inner horn l2 functions as a circularwaveguide, with the radiating aperture 18 at the forward end. Aperture20 is at the forward end of a waveguide using the outer surface of hornl2 and the inner surface of horn 14 as conductors. Aperture 22 is at theforward end of a waveguide formed by the outer conductive surface ofhorn l4 and the inner conductive surface of horn 16.

To obtain the various radiation patterns, each horn has a plurality ofquadrantally position feeds. Inner horn 12 has two feeds 40 and 42positioned degrees apart. Intermediate horn 14 has four feeds 44, 46, 48and 50, and outer horn 16 has four feeds 52, 54, 56 and 58. Each of thefeeds is actually a transformer or transition of conventional type,making the transition from a coaxial connection 34 to the wave guidehorn. A coaxial conductor 34 extends from each feed at the rear of theantenna, with a coupling 36 for connection to associated transmit orreceive apparatus. The specific configurations of the feeds and coaxialconnections will depend on the range of frequencies being used, thearrangment and structure being well known.

All the antenna horn sections operate in the TE mode, rather than theconventional TEM mode. Second and higher order linear modes areprevented by using waveguide sizes which are too small to support them.The modes of interest are TE and TE In TE, mode the cutoff wavelength lt3.4la in a circular guide, and ll 4.64a in a coaxial guide. In T5 modethe cutoff wavelength A, 2.057a in a circular guide, and A 2.35a in acoaxial guide. These cutoff wavelengths correspond to a ratio betweenouter to inner coaxial diameters of 2:1, and will change only slightlyas the ratio is changed.

The operational bandwidth (BW) for a concentric horn is approximated bythe ratio of the two cutoff wavelengths. Thus for a circular guide:

and for a coaxial guide, the ratio is 1.98

The number (n) of individual horns in a concentric assembly necessary tooperate over a total bandwidth (EBW). for a coaxial waveguide is:

n=LogZBWlLog 2 Maximum bandwidth for a given number of circularlysymmetric horns is achieved with the inner horn operating in thecircular guide TE mode.

Circularly symmetric modes are avoided by the method of excitation,which utilizes a dual coupling technique to ensure a TE mode excitation.Diametrical feeds in a horn are fed in or out of phase to generate alinearly polarized wave, with either a sum or difference pattern. Byconnecting two diametrical feeds to a suitable hybrid, an amplitudemonopulse pattern can be produced in the plane through the two feedpoints. A second set of feeds in quadranture relation will provideorthogonal or conjugate linearly polarized modes. With two conjugatemodes, any desired polarization can be radiated.

A typical arrangement is illustrated in FIG. 5. Diametrically opposedfeeds 54 and 58 of outer horn 16 are coupled through phase shifters 60and 62, respectively, to a hybrid junction 64, which is connected to anassociated signal source. Feeds 52 and 56 are similarly coupled throughphase shifters 66 and 68 to a hybrid junction 70, which is connected toa signal source. Feeds 44-50 in horn 14 would be connected in a similarmanner. In horn 12, the feed 40 would be excited for horizontalpolarization and the feed 42 for vertical polarization.

The signal source may be any suitable microwave transmitter and/orreceiver means, operating on a different frequency for each horn. Theinner horn operates at the highest frequency and the outer horn at thelowest frequency of the useful range, the coaxial horn arrangment makingit possible to have a bandwidth of several octaves. The antenna can beused as a direct radiator or, as illustrated in FIG. 4, can be mountedon a suitable support 72 to illuminate a reflector 74.

With reference to FIG. 5, if feeds 54 and 58 are excited out of phase avertically polarized wave is radiated. Similarly, if feeds 52 and 56 areexcited out of phase, a horizontally polarized wave is radiated. Ifdiametrical pairs 52-56 and 54-58 are fed with equal amplitude in phasequadrature, a circularly polarized wave is radiated.

FIGS. 6a-6d illustrate various specific examples of radiation patternswith the relative phases of the various feeds indicated by directionalarrows. In FIG. 6a, the feeds are in phase in each diametrical pair andthe two pairs are in phase, resulting in a vertically polarizedradiation pattern. In FIG. 6b, the feeds are in phase in eachdiametrical pair, but the two pairs are I degrees out of phase,resulting in a horizontally polarized radiation pattern.

FIG. 60 indicates that the feeds are I80 degrees out of phase in eachdiametrical pair, but the two pairs are in phase. This produces amonopulse difference pattern with horizontal polarization in the azimuthplane. and vertical polarization in the elevation plane. In FIG. 61] thefeeds are I80 degrees out of phase in each diametrical pair and the twopairs are also I80 degrees out of phase, The result is a monopulsedifference pattern with vertical polarization in the azimuth plane andhorizontal polarization in the elevation plane.

If only one polarization is required for a specific purpose onlydiametrically opposed feeds are required, rather than the quadrantalarrangement. The wide band and variable radiation patterncharacteristics make the antenna very versatile, and the compactstructure makes it adaptable to a variety of installations.

- Having described our invention, we now claim:

1. A coaxial horn antenna, comprising:

a plurality of at least three progressively sized tubular horns securedtogether in coaxial alignment and defining concentric radiatingapertures at one end,

the innermost horn having a pair of feeds coupled thereto that arespaced and the intermediate and outermost horns having four feedscoupled that are spaced 90, each of said feeds having means forconnection to a signal source,

said horns are axially staggered with the innermost horn extending apreset distance from said one end, and the intermediate horn extendingan intermediate distance between the innermost horn and the outermosthorn,

each of said feeds includes a coaxial to wave guide transition elementcoupled to the inner surface of the respective horn,

said means for connection comprising a coaxial conductor to saidtransition element,

and the ratio of cross sectional size between adjacent horns isapproximately 2 to l.

1. A coaxial horn antenna, comprising: a plurality of at least threeprogressively sized tubular horns secured together in coaxial alignmentand defining concentric radiating apertures at one end, the innermosthorn having a pair of feeds coupled thereto that are spaced 90* and theintermediate and outermost horns having four feeds coupled that arespaced 90*, each of said feeds having means for connection to a signalsource, said horns are axially staggered with the innermost hornextending a preset distance from said one end, and the intermediate hornextending an intermediate distance between the innermost horn and theoutermost horn, each of said feeds inCludes a coaxial to wave guidetransition element coupled to the inner surface of the respective horn,said means for connection comprising a coaxial conductor to saidtransition element, and the ratio of cross sectional size betweenadjacent horns is approximately 2 to 1.